Linear suspension dampers, such as shock absorbers and McPherson struts, may include a rod and piston moving within a fluid-filled housing. Suspension movements transmitted to the rod and piston may be dampened as the damper compresses and rebounds. Dampening forces are generated within the housing by fluid friction forces that oppose the movement of the rod and piston.
Current damper designs may include a magnetorheological (MR) fluid that allows the generated dampening forces to be selectively controlled. MR fluids are generally suspensions of magnetic particles such as iron or iron alloys in a fluid medium. The flow characteristics of these fluids can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to suspension of the particles. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. MR fluids are well known and have been found to have desirable electromagnetomechanical interactive properties for controlling dissipative forces along the damper's axis.
A linear acting MR damper piston may include a coil assembly, a core, and an annular piston ring positioned around the pole pieces to form an annular flow passageway. When the piston is displaced, MR fluid is forced through the passageway from one area of the damper housing to another. When the coil is energized, a magnetic field permeates a portion of the passageway and excites a transformation of the MR fluid to a state that exhibits increased damping force (i.e., the MR fluid viscosity is increased). The amount of dampening force may be selectively controlled by adjusting the current run through the coil assembly (e.g., the damper “on-state”).
The damping performance of a suspension damper is largely dependent on the force-velocity characteristics of the damper. In standard suspension dampers of the prior art that do not use MR fluid, the force-velocity curve typically has a steeper slope at low velocities and desirably passes through the zero point of damping force at zero velocity, thus producing a smooth transition between damper movements in compression and extension directions. Without special design considerations, however, a suspension damper using MR fluid tends to have a force-velocity curve that intersects the force axis at a value above zero from the positive velocity side, and a value below zero from the negative velocity side, thus producing a jump in force between finite positive and negative values with each change in the direction of damper movement. These jumps in force tend to provide harshness to the vehicle ride which may be felt by the vehicle occupants.
To address this problem, a known design of a linear acting MR damper includes a piston having a substantially annular, magnetically energizable gap formed between a piston ring and core, and one or more magnetically non-energizable bypass ports positioned radially inward from the gap. The bypass ports provide means for allowing fluid flow through the piston that is not influenced by MR state transformations. As such, the bypass ports may prevent unwanted damper performance characteristics during on-state and at low temperatures. Accordingly, it would be desirable to provide an MR damper piston with the advantages of bypass ports.
In one piston design, the bypass ports extend across the piston core and through corresponding holes formed in flanking piston end plates. This design may have disadvantages. For example, as the piston typically includes a rebound bumper positioned adjacent one of the end plates, the bumper can extrude into the end plate holes during damper operation. This may significantly increase bumper wear-and-tear and failure rate. In addition, the end plate holes require alignment with the bypass ports and also weaken the end plate structure thereby increasing failure rate. Accordingly, it would be desirable to provide an MR damper piston that does not require hole(s) in the rebound bumper-side end plate.
Desirable damper performance usually requires that significantly less dampening forces are generated during a compression stroke as compared to a rebound stroke. Switching the MR fluid state may provide such “asymmetric” dampening forces. However, using MR state transformation to generate asymmetric dampening forces may have disadvantages. For example, a substantial amount of total MR dampening capacity may be used to generate rebound stroke dampening forces. The ability of the damper to handle finely-tuned dampening or other events may thus be diminished. Accordingly, it would be desirable to generate asymmetric dampening forces without the need for MR state transformations. The MR dampening capacity could thus be preserved to handle events requiring additional dampening force and for other circumstances.
Therefore, it would be desirable to provide a magnetorheological piston and damper assembly that overcomes the aforementioned and other disadvantages.